A conventional cathode ray tube (CRT) includes a phosphor-coated front panel, a funnel section housing an electron gun for generating one or more electron beams, and a steel color selection electrode (referred to hereinafter as a mask) for exposing selected phosphor elements to the electron beams. The mask is ordinarily welded to a frame which supports it within the front panel and sustains it in a position essentially parallel to the phosphor coated screen. The combination of the mask and its supporting frame, or the mask itself where it is of the type not requiring a separate supporting frame, will be referred to herein as a mask assembly.
In the construction of CRT's, one of the first maskrelated operations is to bake the mask assembly in an environment having a low oxygen content and a dew point of approximately 85. By so baking the mask assembly, its surface is given a dark, adherent oxide coating which will protect the mask assembly from corrosion as it and other tube components are processed and assembled in a factory.
It is not unusual for the production line processing of a mask assembly to take 8 hours or more. During this processing interval, flaky oxide deposits can, in the absence of a protective coating, form on the surface of the mask assembly. Such deposits are easily rubbed off and may find their way to the mask apertures where they may lodge and plug some of the apertures.
A dark coating also serves to make the mask assembly a better radiator of heat in order to keep thermal expansion of the mask to a minimum. Although shadow masks are generally provided with some form of thermal compensation to minimize misregistration between mask apertures and the screen's phosphor elements as mask expansion occurs, such compensation is never perfect. Therefore, the better radiator a mask is, the less misregistration will occur as the mask heats up under electron bombardment.
In addition, a dark, preferably black mask will absorb light which finds its way into the tube rather than reflecting it onto the screen where it may tend to dilute or "wash-out" the picture.
For one type of color CRT presently enjoying commercial success, the Chromacolor (trademark of Zenith Radio Corp.) tube, the processing of the tube components proceeds as follows. The inner side of the front panel or screen is coated with a dark, light-absorbing material in which there are patterns of openings in which red, blue and green phosphors will be deposited.
A photoresist coating which includes green phosphors, for example, is then applied over the dark coating. The screen is then exposed to a source of light directed through the mask apertures to render insoluble those areas of the photoresist which occupy the holes in the black coating in which green phosphor is to lie. The screen is then developed to rinse away the green photoresist everywhere except in those areas rendered insoluble by the exposure.
This process is then repeated for the red and blue phosphors so that a pattern of red, blue and green phosphor triads are deposited in the surrounding black material.
The next step involves enlarging each mask aperture so that its dimension in any direction in which misregistration can occur between an electron beam landing and an impinged phosphor element is greater than the corresponding dimension of the impinged phosphor element by a predetermined amount. This will insure that the electron beam which passes through a mask aperture is large enough to cause the electron beam landing to be larger than the phosphor element which it illuminates. This "negative guardband" principle, in combination with the concept of including a light-absorbing material between adjacent phosphor elements, is fully disclosed and claimed in U.S. Pat. No. 3,146,368, issued to Fiore et al, and assigned to the assignee of this invention.
A tried and proven method of enlarging the mask apertures is to re-etch the mask with ferric chloride until a predetermined degree of aperture enlargement has occurred. However, before the etchant can react with the mask, the original protective oxide coating on the mask which has provided corrosion protection during the preceding processing steps must first be removed therefrom. This is usually accomplished by spraying a chemical solution onto the mask assembly to completely strip away the protective coating.
When the re-etch of the mask is complete, the mask assembly will require another protective coating. This coating cannot now be provided in the way the original coating was generated, that is, by baking the mask assembly at a very high temperature. At this point, exposing the mask assembly to the elevated temperatures required to produce such a coating would deform the mask and ruin the registration between the mask and the pattern of phosphor elements on the CRT screen.
Prior art techniques of generating this final coating have included applying chemical solutions such as a standard iron-phosphating solution to the re-etched mask. However, the next processing step, frit sealing the front panel to a mating funnel and baking the assembled tube to cure the frit, tended to adversely affect the mask coating. Since the mask assembly is installed within the tube, it is, of course, subject to the same baking time and temperatures as the curing frit. As a result, the prior art mask assembly coating tended to deteriorate under the intense heat (about 435.degree.C). This caused the mask coating to become flaky and only marginally adherent. The coating could then fall off or be rubbed off the mask assembly and plug some mask apertures or increase the possibilities of arcing within the tube. Accordingly, it is apparent that there is a need for a method of providing an improved coating on CRT mask assemblies, particularly, but not exclusively, for mask assemblies which undergo the above-mentioned re-etch process.